1. Field of the Invention
The present invention relates to a method of growing a zinc-oxide-based semiconductor and a method of manufacturing a semiconductor device, and particularly to a method of growing a zinc-oxide-based semiconductor layer by an MOCVD method and a method of manufacturing a semiconductor light emitting device using the method.
2. Description of the Related Art
Zinc oxide (ZnO) is a direct transition semiconductor having band gap energy of 3.37 eV at room temperature and expected to serve as a material for optical devices of a blue to ultraviolet range. In particular, the binding energy of excitons thereof being 60 meV with refractive index n=2.0, ZnO has physical properties extremely suitable for semiconductor light emitting devices. Further, not being limited to light emitting and light receiving devices, ZnO can be widely applied to surface acoustic wave (SAW) devices, piezoelectric devices, and the like. Moreover, ZnO has features that its raw materials are inexpensive and that it is harmless to the environment and human bodies.
Generally, an MOCVD (Metal Organic Chemical Vapor Deposition) method and an MBE (Molecular Beam Epitaxy) method are being used as the crystal growth method of a zinc-oxide-based compound semiconductor. The MBE method is a crystal growth method performed under an ultrahigh vacuum, and hence there is the problem that the apparatus for it is expensive and that productivity is low. In contrast, for the MOCVD method, the apparatus is relatively inexpensive, and large area growth and multi-wafer simultaneous growth are possible. Thus, the MOCVD method has the advantages of high throughput and being excellent in mass productivity and cost.
A zinc oxide crystal growth method using the MBE method wherein a low-temperature ZnO layer showing single-crystal characteristics is grown on, e.g., a sapphire substrate and then flattened by heat treatment at a high temperature and thereafter a high-temperature ZnO layer is grown to obtain a ZnO layer good in crystallinity is disclosed in, e.g., Japanese Patent No. 3424814 (Reference 1). More specifically, a low-temperature grown ZnO single-crystal layer formed at a growth temperature lower than a crystal growth temperature at which a ZnO single crystal is generally grown, is disclosed. However, the method disclosed in Reference 1 is a growth condition/method effective only for the MBE method and cannot be applied to the MOCVD method. That is, the growth conditions for the MBE method, where crystal growth in non-stoichiometry conditions is possible, cannot be applied, as it is, to the MOCVD method as well known (e.g., Japanese Patent Application Laid-Open Publication No. 2005-340370 (Reference 2)). Hence, methods of growing a single-crystal layer of a zinc-oxide-based semiconductor using the MOCVD method are being actively studied.
Various methods of growing zinc oxide (ZnO) or a zinc-oxide-based semiconductor on a substrate of another material such as sapphire (Al2O3) by the MOCVD method have been disclosed in, e.g., Reference 2 and Japanese Patent No. 3859148 (Reference 3). For example, Reference 2 discloses that micro crystals of MgZnO are formed as a preliminary buffer layer using gas O2 as an oxygen source on an A-plane sapphire substrate or a C-plane silicon carbide substrate and that with the micro crystals as seed crystals, a MgZnO crystal is formed as an actual buffer layer entirely over the substrate. Reference 3 discloses that a low-temperature formed polycrystal or amorphous laminate is annealed at a high temperature so as to be a buffer layer. However, with these methods, when the polycrystal is single-crystallized by heat treatment, defects are left between adjacent crystal grain boundaries, and when the actual buffer layer is grown from the micro crystals with grains merging, defects are left. Hence, it is difficult to greatly reduce the number of crystal defects. As such, to date, attempts to improve crystallinity after forming a buffer layer of a polycrystal, micro crystals, or amorphous material have been made, but the method to improve crystallinity for the MOCVD method is complex, and it has been difficult to grow a ZnO-based crystal of high crystalline quality on a substrate.
As described above, methods of growing a ZnO-based single crystal on a sapphire substrate or the like using the MOCVD method have been proposed in large number, but they fall short of being a method of growing a ZnO-based single crystal of high crystalline quality in a simple, convenient way.
Further, in the case of the MOCVD method, if crystal growth is performed in the environment of a temperature at which crystallinity is improved (about 600° C. or higher), the ZnO-based single crystal tends to become a (hexagonal) columnar crystal, a mesh-like crystal, or a (hexagonal) disc-like crystal which is oriented in a c-axis direction. With this polycrystal or imperfect single crystal strongly oriented in a crystal axis direction, grain boundaries and dislocations cause a leak current or local current concentration in semiconductor devices, resulting in degradation of device characteristics and device lifetime. In particular, in semiconductor light emitting devices, leak currents and current concentration result in degradation of characteristics such as light-emission efficiency and device lifetime. Further, the crystal surface being not even or flat results in a decrease in process accuracy in semiconductor processes such as lithography and etching and a decrease in production yield, and also results in a decrease in production yield in cleavage, breaking, and the like.
As such, to date, it has been difficult to grow a ZnO-based semiconductor single crystal which is flat and has a small number of grain boundaries and dislocations, on a substrate of another material such as a sapphire substrate by the MOCVD method. In order to make semiconductor devices, especially semiconductor light emitting devices driven by large operating current density, higher in performance and reliability, it is extremely important to develop a method of growing a crystal close to an ideal crystal, which has a small numbers of crystal defects or low defect density and is flat.